This invention relates to switching in packet-based or frame-based data communication systems, and particularly to the control of a look-up process in such switches.
In a frame-based data communication system, wherein packets or frames of data are conveyed and directed under the control of, among other things, source addresses and destination addresses represented by respective data fields within the frames, it is customary to provide in a switch a database which relates source addresses obtained from frames received by the switch to other data, including a number identifying the port by which the frame was received at the switch, and preferably also the age of the relevant entry in the database. This database, customarily called xe2x80x98forwarding databasexe2x80x99 is established in order to facilitate the dispatch of frames, which are usually temporarily stored in the switch, from the appropriate port or ports. Such a forwarding database is built up by a process of learning source addresses. In particular, when a frame is received by the switch, the source address is extracted from the frame and is entered into the database against the port on which it was received. Then, when some other frame arrives at the switch, destined for that particular address, the database is searched and the frame is dispatched (after queueing and arbitration if appropriate) on the port associated with that particular source address. If a frame arrives at the switch and the destination address in that frame is not found in the table of source addresses and ports, it is customary, and prescribed by transmission standards such as IEEE 802.1d, to broadcast the frame to all ports of the switch. In order to limit the incidence of broadcast traffic, it is very desirable that a forwarding database should be maintained up-to-date by a process of xe2x80x98learningxe2x80x99 all source addresses identified in frames arriving at the switch.
Ideally, for every frame received by the switch, two look-ups are performed. A necessary look-up in the forwarding table is a destination address look-up in order to determine where to send the frame. The second look-up is to determine whether the source address in a received frame is already in the forwarding database and, if not, to execute a write operation in order to make the necessary entry of the source address and the port on which the respective frame has been received.
A forwarding database is typically stored in a memory external to a switching basic owing to the substantial capacity (memory space) required for a forwarding database. The bandwidth available for operations relating to the memory depends partly on the speed at which the memory is clocked and the width of the database (i.e. the length of words permitted in the database). The bandwidth is therefore limited partly by technological feasibility.
As data rates increase, especially from the rates of ten or one hundred megabits per second customary today to one gigabit per second and beyond, the rate at which frames are received by a switch increases accordingly. The bandwidth available for operations relating to the memory may accordingly be insufficient to perform both a destination address look-up and a source address look-up for every frame received by the switch.
The present invention particularly therefore relates to the management of the rate at which source address look-ups are performed to ensure that the forwarding database is adequately maintained while performing destination address look-ups for every frame and allowing the transmission and forwarding of frames at xe2x80x98wire-speedxe2x80x99 (i.e. the data transmission rate) between network devices.
The invention is based on a process according to which the enabling of a source address look-up is dependent on the availability of a token. Preferably the supply of such tokens is made at a controllable rate. In a further aspect of the invention, the writing of an entry of a source address in the database is made dependent on a comparison of the age allotted to an existing entry in the database and a current indication of age.
Further features of the invention will be apparent from the following description with reference to the drawings.